1. Field of the Invention
This invention relates to a device and method of cleaning protective screens used in a Selective Catalytic Reduction (SCR) system.
2. Description of the Related Art
Selective Catalytic Reduction (SCR) systems are increasingly being applied to coal-fired power stations to reduce nitrogen oxide (NOx) emissions. SCR systems commonly include a SCR reactor that contains a NOx reducing catalyst that converts NOx present in flue gases emitted from a combustion source into by-products of nitrogen and water. Many power station installations place the SCR reactor system in a “high dust” location between the combustion source and a particle collection system. Generally, these installations have ductwork that directs or diverts the particle-laden flue gases from the combustion source to the SCR reactor system and then to an air preheater.
The dust loading ability of these SCR systems, located in such “high dust” locations, is an important consideration in their design and use. In particular, the NOx reducing catalyst composition and construction thereof should be designed to withstand erosion and potential chemical degrading effects of the fly ash and other particles in the flue gases. Similarly, the ductwork to and from the SCR reactor system and the associated internal structures within the SCR system should be designed to withstand this erosive environment. For example, certain aspects of the ductwork design parameters, such as the duct's gas velocity, may be closely monitored to insure proper operation. In particular, undesirable operating results such as unwanted fly ash drop out should be prevented or minimized by selection of proper operating design parameters.
The NOx reducing catalyst construction in the SCR reactor also requires proper design considerations. Generally, the NOx reducing catalyst is constructed in a manner that has gas channels whereby the flue gases can pass through such channels to maximize contact with the catalyst surface thereby maximizing the reduction of NOx. The gas channels of the NOx reducing catalyst typically have a diameter in the range of about 5 to 7 mm. However, particles in the flue gas (hereinafter referred to as “fly ash”) generally have a wide range of sizes (e.g. from 1-2 microns up to 7 mm and larger).
The larger particles of fly ash, sometimes referred to as “popcorn ash” or large particle ash (“LPA”), may pose problems with the NOx reducing catalyst. For instance, when the gas channel diameter is 5-7 mm and the fly ash particles are larger than 7 mm, the large fly ash particles may lodge within the channels and block the flow of flue gas through the catalyst. Even fly ash particles smaller than 7 mm have been shown to plug the catalyst channels because of the irregular shape of those particles. If just one irregular shaped fly ash particle gets lodged in the catalyst channels, other fly ash particles cannot pass through the channel, thereby blocking the channel.
This blockage decreases the overall NOx reduction capability of the system because once a gas channel is blocked, that reaction channel in the NOx reducing catalyst becomes ineffective. Once many reaction channels become blocked, fly ash accumulation on the NOx reducing catalyst surface increases rapidly. Over time, the surface of the NOx reducing catalyst can eventually become so covered with fly ash that the SCR system cannot meet its NOx reduction target. Also, the resulting increase in catalyst pressure drop will require the system to be cleaned. For SCR units without a gas bypass capability, this build-up may require the combustion source to be shut down as well.
A known practice to mitigate this ash or dust build-up over the NOx reducing catalyst has been to place one or more mesh screens over the NOx reducing catalyst. The openings in the mesh screens are selected to be slightly smaller than the diameters of the channels in the NOx reducing catalyst. Thus, large fly ash particles are stopped from entering the channels in the NOx reducing catalyst. While this method can keep the actual catalyst channels clean, its ability to lengthen the time between outages for cleaning is uncertain. Cleaning is still necessary for this method because the quantity of large fly ash particles entering the SCR reactor remains unchanged and these fly ash particles are now collected on the screens instead of on the catalyst or within its channels. Large fly ash particles may accumulate on the screens, thereby creating blockages which will then start collecting smaller fly ash particles. It is therefore possible to have mounds of fly ash on each screen.
Mounds of fly ash that are collected on the screens can significantly increase the pressure drop across the SCR system and may lead to localized areas of high velocity, which have been known to cause erosion within the catalyst. The accumulation of fly ash on the screens will also affect gas distribution and gas velocity into the NOx reducing catalyst. This in turn will reduce the efficiency of the SCR system.